In the known art, for example, a bypass conduit (bypass circuit) is connected in parallel to a fluid system, in which a generator is activated through a water turbine. By controlling a flow rate control valve, which is installed in the bypass conduit, a flow rate at a water supply side is controlled to be constant (see, e.g., Patent Document 1). In the example of Patent Document 1, a flow rate detector (flowmeter) is provided at the water supply side. The frequency of the water turbine is controlled based on the flow rate detected with the flowmeter.